1. Field of the Invention
This invention relates to radio broadcasting and, more particularly, to a method of and apparatus for recovering an underlying digital broadcast signal from an amplitude-modulated broadcast carrier and for improving the signal-to-noise ratio in analog amplitude modulated receivers, especially at higher audio frequencies.
2. Description of the Related Art
There has been increasing interest in the possibility of broadcasting digitally encoded audio signals to provide improved audio fidelity. Several approaches have been suggested. One such approach, set forth in U.S. Pat. No. 5,588,022, which is assigned to the assignee hereof, and which is incorporated herein by reference, teaches a method for simultaneously broadcasting analog and digital signals in a standard AM broadcasting channel. An amplitude-modulated (AM) radio frequency (RF) signal having a first frequency spectrum is broadcast. The amplitude modulated radio frequency signal includes a first carrier modulated by an analog program signal. Simultaneously, a plurality of digitally-modulated carrier signals are broadcast within a bandwidth which encompasses the first frequency spectrum. Each of the digitally-modulated carrier signals is modulated by a portion of a digital program signal. A first group of the digitally-modulated carrier signals lies within the first frequency spectrum and is modulated in quadrature with the first carrier signal. Second and third groups of the digitally-modulated carrier signals lie outside of the first frequency spectrum and are modulated both in-phase and in-quadrature with the first carrier signal. Both transmitters and receivers are provided in accordance with that method.
The waveform in the AM compatible digital audio broadcasting system described in the above referenced U.S. Pat. No. 5,588,022, has been formulated to provide optimal data throughput for the digital signal while avoiding cross-talk into the analog AM channel. Multiple carriers are employed by means of orthogonal frequency division multiplexing (OFDM) to bear the communicated information.